Making the RV Significantly Colder – An Air Conditioner Mod

A couple days ago, Stef offhandedly posted a picture of me tinkering with the RV air conditioner on our Facebook page. Nothing new, she posts loads of pics of what we’re up to day to day there, and many of them involve me working on Lance. I didn’t think much about it, but apparently, many of you left comments and were interested in what I did and if it worked. The quick answer is: I adjusted the thermostat and yes it worked.

But before you go trying this yourself, here’s the situation I had:

Our new air conditioner, a Dometic Penguin II, works very well and produces a temperature differential (intake vs. cold air) of about 20 degrees. So the air conditioner was not “broken”.

With the thermostat (the colder/warmer knob) turned all the way cold, the air conditioner would cool the inside of the RV to about 76 degrees.

Once the temperature was 76 degrees, the compressor on the AC would stop running. The fan would still run, but it was no longer cooling the air. When it warmed back up to 78, the compressor would kick on again and cycle with the temperature.

In my mind, 76 degrees is not “coldest”. It’s not even “cold”. In fact it’s uncomfortable. If you’re sitting on the Ultraleather dinette next to a window in the front of the rig, and the temperature is at 76 in the back of the rig – you’re going to be sticking to the seats. Not cool (ha!). So, I decided to change the air conditioner’s minimum set point from 76 degrees to something considerably lower.

A Word of Warning

The air conditioner manufacturers have the minimum temperature set in the mid-70s for a reason. They don’t want your air conditioner to “freeze up”. That could prevent the AC from working, damage the air conditioner, or lead to a service or warranty call. They don’t want any of these to happen, so they deploy their air conditioners at temperatures that will make you sweaty – but will guarantee that the unit never freezes up.

Back in the 1600s, they had air conditioners that froze up.

I’ve always regarded the “air conditioner freeze up” as a myth. I think it’s the same kind of reasoning that gives us RV shower heads that leak all the time on purpose – because there’s an extremely remote chance you could scald yourself if it doesn’t. (By the way, I fixed that problem too, and you can read all about that here.)

Of course, I should point out that we live in Utah, where the relative humidity typically hovers in the single digits. We don’t even get condensation on the outside of an icy glass. So the chances anything would “freeze up” on me in the middle of summer are basically zero. If you live elsewhere, you may want to take that into account.

But back to the AC. There are four main reasons your air conditioner may freeze up:

Insufficient airflow across your air conditioner’s evaporator coil.

Your air conditioner is low on Freon.

You are running your air conditioner when the outside air is below 62 degrees.

There is a mechanical failure (kink in refrigerant lines, a blow fan is not running, etc.)

Number 1 is something you can prevent by keeping your air conditioner’s intake screens clean, so put that on your routine maintenance checklist. Numbers 2 and 4 represent problems that I’d like to know about – and a frozen air conditioner seems like a good indicator. And number 3 is something I’m not worried about because I’m not stupid. So even if I did believe in air conditioner freeze-ups and Bigfoot, I think the problem is manageable at best, and just an indicator of a bigger problem at worst.

What I Did

So, if you think you have the same problem with cycling that I did; you think your air conditioner is unlikely to freeze up; you don’t have a separate wall thermostat for the air conditioner; you can use a screwdriver; and you’re unafraid of possibly voiding your AC warranty, here’s what I did. I have tested this procedure on my own AC unit, which is a Dometic. I suspect that others (Coleman, etc) may be similar, but I have not verified this.

First, obviously, make sure that ALL POWER to your coach is turned off. No generator. No shore power. No inverter. And I even shut the batteries down for good measure.

Then, remove the indoor air distribution box (ADB) cover. You’ll wind up with something on your ceiling that looks more or less like this:

What you’re looking for is the inside part of the “colder/warmer” knob – the thermostat. It will likely be inside another metal enclosure inside of the ADB. In the picture above, I’ve loosened the cover on this interior box. If you’re stumped, it will have a strand of copper leading into it. The other end of that copper is on a probe in the intake air.

Once you’ve located the thermostat, do what you need to in order to remove it from the ADB. In my case, I had to remove two screws. But your air conditioner may have nuts or some other way of holding it still.

With the thermostat removed, you’re looking for an adjusting screw of some sort. It probably won’t be labeled, because they don’t want you messing with it. But it should be there. On my Dometic, the screw looked like this:

Now, for the tedious part. In small increments, start adjusting the screw. You want to do this slowly:

Adjust the set point screw ¼ turn. (I don’t know which direction on yours!)

Reassemble the ADB box (Safety first! You’ll be turning on the power.)

Run the air conditioner until the compressor cycles

If it gets cold enough for you, you’re done. If not – lather, rinse, repeat.

In the end, I gave my screw about one and a half turns clockwise. When I had finished, the “warmest” setting on the thermostat gave me about 80 degrees at the dinette, and 68 degrees at the dinette was at the ¾ mark.

It’s important that there is *some* realistic temperature where the air conditioner will cycle off, and that temperature is at 62 degrees or above. So don’t overdo it!

And with that, I was done.

This took about 15 minutes to figure out, and about two hours to do with all the setting and checking.

James is a former rocket scientist, a USA Cycling coach, and lifelong fitness buff. When he's not driving the RV, or modifying the RV (or - that one time - doing both at once), you can find him racing bicycles, or building furniture, or making music. In his spare time, he works for a large IT company.

We did the same thing and it seemed to work. I unfortunately turned too much the 1st time and the screw came out of the spring. No big deal we ordered another. Question is do I have to rotate knob either way hot or cold direction prior to turning screw or just give it a turn and repeat? I saw another YT vid where a guy used a meter etc and turned the knob to a certain point first. Necessary or no? Installing the new one today.

Correct. Our rigs do not have wall-mounted thermostats. The interior air distribution box will be different on a unit with a wall thermostat. As I don’t have one to look at, I can’t say if the procedure here would work or not. You’d have to open yours up to see if the mechanics were similar.

I adjusted my 13,500 btu dometic using your suggestions, exact same thermostat, same amount of turns. I am able to get the inside of my unit to 62° which definitely will help during the summer here in Georgia.

Thanks for this great idea and explanation!. I ended up not having to do this to my air conditioner, however I am curious to see if you think you can add a wall thermostat to a non ducted dometic air conditioner. Got any ideas on that?

Well, I think it might be possible, but there would be new parts to order. Most of the manufacturers make a “ducted” ceiling box assembly – that assembly won’t have controls, and will have the ability to plug in remote controls. So yes, I think it’s possible. Worth it? I can’t say.

The only danger I’m aware of with air conditioners is the mythological danger of them freezing up. I’ve never seen it, but people say it happens. (People also say they’ve seen the Loch Ness Monster.) Anyway, in theory, freezing up an air conditioner would require moisture in the air. If you’re in Vegas… I’m pretty sure moisture in the air isn’t a concern. In your situation, I’d crank it. Max cool. Max fan.

Hey James, This is just what I was looking for. I have the same exact unit that you do. I have another question. Mine has a heat pump and this is used in my Sprinter Van so no furnace. My unit will not even come on at 42 degrees and below. Most people I have asked, have no idea because they have a furnace and they do not turn the unit on when it is below 42. Of course, I am trying to use the heat pump. Do you know of anyway bypassing this 42 degree system shutdown? Thanks David

James, I purchased a used Coleman Mach 3 13,500 with a new Coleman inner unit assembly (fan & thermostat controls). Installed assembly….the blower is working but the system is not cooling (70 degrees @ discharge (I don’t believe the compressor is cycling)). The outer unit appears to be in good shape & clean (no signs of leaks). The previous owner stated it worked good but, he replaced with a larger unit. Now, I may be creating my own issue…… outside temp was in mid 60’s when I was installing/ testing the unit. Does the outside temp have to be a certain degrees before the system will actually start cycling (to prevent possible freeze up)?

I just installed a dometic and it was not getting cold., Pulled mine apart like you showed and went 1.5 turn clockwise and was fine til i put the cover back on… I had to bend the actual thermostat away from the plastic and more out to the intake… Now its working perfectly…. So those 2 issues fixed it…

When running my AC’s, I put the fans on manual speed, normally at med. and the compressor is the only item that cycles. This takes start up load off the gen set. And keeps the ac from ever freezing up. I normally keep my thermostats at 70 degrees.

My AC (Coleman Mach III 13.5) will kick on, begin to cool but after 30sec to a min or so, stop cooling but fan will blow. Generator nor breaker tripped. Temp outside & inside upper 80’s – 90ish. Then have to play with wall t-stat to get it to kick back on. Then happens all over again. Thoughts?

It certainly sounds like a thermostat issue. But as we don’t have a wall thermostat, I’m at a loss to tell you how to troubleshoot. In your place, I’d probably install a similar thermostat temporarily and see if that fixed it. Maybe a friend with the same model? Or buy a new one from a parts dealer?

My original dometic AC on 94 airstream was great. We live in the south. The control board fried after 15 years. The cooling fins on the condenser were corroding off from being on the beach so wasn’t worth fixing. As a temporary fix I straight wired the unit to come on high only at max cool. Used a small ceramic heater as the thermostat control. This temp fix worked great for 6 years. Had the dealer install a brand new one. It sucks! Always freezes up and never gets cold. It doesn’t have a freeze sensor. Several dealers and call to Dometic confirm it is oporating as designed. Bad design! Suffered and cussed for 3 years. Installed a percentage timer switch between the thermostat and compressor to defrost. Adjusted the thermostat per your instructions. Got my cool happy camper back! External adjustable timer is mounted on ceiling next to ac. Setting on 93% at 15 minute cycle.

Glad you’re cool and happy again! I’ve heard that from several folks… that the newer air conditioner units simply don’t work as well as older ones. Some blame more environmentally-friendly designs and refrigerants. There may be something to that, but I’ve not seen an official statement from any AC maker to that effect.

Hey there! Our camper wasn’t blowing cold air this morning. It was also making a humming/buzzing noise that goes on and off while it’s on. We are definitely in humid weather so don’t think that it’s freezing up. What do you think could be wrong with it?

Several questions needed to get started on this one. What brand and model is your air conditioner? Is there no air at all blowing (fan issue), or is there air blowing, but it’s just not cold (compressor issue). If you let the unit sit idle for a whole day, will it then start up?

I found all of this info interesting. Basically the air coming out of my RV AC is no where near as cols as the air coming out of my home or automobile AC. I assumed this was a low on freion issue. My RV is ionly 3 years old. I have a wall thermostat, could it just be low on freon or do you think it could be the same issue you identified with the internal AC aetting?

I would guess the problem is NOT low freon, but instead the internal thermostat setting discussed in this post. Just a guess, but I think regardless of what your wall thermostat says, an internal regulator on your AC unit will only let it get so cold. It’s simple enough to check, and could save you a trip to the service center.

James, we have a thermostat controlled AC in our 5th wheel, but seem to have the same problem you did…it won’t ever get below 77 deg unless it is lower than that outside. Even when it is only 82-84 outside in SoCal, 77 is the lowest it will go. What do you think I should look for in our unit, Coleman Mach 15? I’m assuming to trace the thermostat wires into the unit and find something similar?

Hmmm… That depends on if the limiting factor is in the programming of the thermostat itself, or somewhere up in the AC unit. I’d say your guess is as good as any. If you’re feeling adventurous, go for it. One thing you might try is to get a wiring diagram for the wall thermostat, and then look for the wires that should be connected or grounded when the wall thermostat is calling for cooling. Then connect or ground those as appropriate (effectively bypassing the wall thermostat), and see if the air conditioning gets colder than 77. If it does, then you know the problem is in the wall thermostat. If it doesn’t, then you know that there is something in the AC unit itself that is preventing it from working properly.

James I have a roof top unit with thermstat control on wall. When unit cools down an shuts off , problem begins when trys to come back on. It will click , click again come on, shut off and repeats until i turn thermostat down colder or run on manual

LOL, that you mention Utah and not freezing up together. The only time I have seen bigfoot, I mean had my AC freeze up, is in Moab, Utah. I swear it was 110 during the day and still 85 at night and it never went off. Took cover off and it was one solid block. Drove out of the park and to the McD’s let it sit in the sun and melt while we sat inside.

Thanks for the tip, but I can tell you your 62F limit is not chiseled in stone. Here in Houston where the humidity is often un triple digits I have frozen up my AC when the temperature is in the upper 70s. I try to get my coach as cold as possible at night to coast through the next day, but a couple of times I froze it up and I can guarantee the outdoor temperature never dropped below 78F

Well, it sounds like you might be one of those who needs to do his own testing to determine an appropriate limit. But… if my AC was freezing up at 78 degrees, I’d be looking for a different problem (low freon, etc) or replacing the unit altogether.

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